(a) Field of the Invention
The present invention relates to a method of producing a high molecular weight epoxy resin which may be suitably used as a material for adhesives, insulating materials, paints, molded articles and films.
(b) Description of the Related Art
The method of producing high molecular weight epoxy resins by employing relatively low molecular weight difunctional epoxy resins and dihydric phenols as polymerization materials is generally called two-stage method, and the first literature on this method is the specification of U.S. Pat. No. 2,615,008 which corresponds to the published specification of Japanese Patent Application filed by the same applicant as the assignee of the U.S. Patent and published under publication No. 28-4494. In these literatures disclosed is a method of producing a higher molecular weight epoxy resin having an epoxy equivalent weight of 5,600 by carrying out a reaction at 150.degree. to 200.degree. C. in the absence of solvents by using sodium hydroxide as a copolymerizing catalyst. The average molecular weight or tile obtained epoxy resin is presumed to be about 11,000. In these literatures, however, there is no example where solvents are used.
An example of the literatures in which the use of solvents is disclosed is the specification of U.S. Pat. No. 3,306,872. Particular examples of the literatures in which the Use of solvents in working examples is disclosed include Japanese Patent Application Kokai Koho (Laid-open) No. 54-52200, Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757, Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757, Japanese Patent Application Kokai Koho (Laid-open) No. 60-144323 and Japanese Patent Application Kokai Koho (Laid-open) No. 60-114324. The solvents used in these literatures include methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, ethylene glycol monoethyl ether and ethylene glycol monomethyl ether. These solvents are classified into ketone solvents and ether (cellosolve) solvents.
In the specification of U.S. Pat. No. 3,306,872, either methyl ethyl ketone or ethylene glycol monomethyl ether is used as a solvent, and the concentration of the solids content is 20 to 60% by weight. The catalysts used therein are hydroxides and phenolates of alkyl metals and benzyltrimethylammonium. The polymerization reaction is continued at a temperature of 75.degree. to 150.degree. C. until the weight average molecular weights of the formed high molecular weight epoxy resins increase to at least 40,000 or more. The average molecular weights of the obtained high molecular weight epoxy resins are measured by the viscosity method to be 50,000 to 1,000,000. However, it is known that, in the viscosity method, the calculated average molecular weights are very dependent on the parameters established for the calculation, and therefore the average molecular weights of the high molecular weight epoxy resins produced In the U.S. Pat. No. 3,306,872 are not entirely accurate.
Another working example wherein a high molecular weight epoxy resin is supposed to be obtained by carrying out the polymerization in a solvent is disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 54-52200, in which it is disclosed that a high molecular weight epoxy resin having an average molecular weight of 45,500 is obtained by using ethylene glycol monoethyl ether as a solvent. Further, it is disclosed in Japanese Patent Application Kokai Koho (Laid-open) No. 60-118757 that high molecular weight epoxy resins having average molecular weights of at most 31,000 are obtained by using methyl isobutyl ketone, cyclohexanone or ethylene glycol monoethyl ether as a solvent. In Japanese Patent Application Kokai Koho (Laid-open) No. 60-144323 disclosed is the production of a high molecular weight epoxy resin having an average molecular weight of 53,200 by tile use of methyl ethyl ketone as a solvent, and in Japanese Patent Application Kokai Koho (Laid-open) No. 60-144324 disclosed is the production of a high molecular weight epoxy resin having an average molecular weight of 66,000 by the use of methyl ethyl ketone as a solvent. In every one of these four literatures, the average molecular weights are measured by gel permeation chromatography, but the measuring conditions and the calculation methods are not disclosed. The molecular weights measured by gel permeation chromatography vary largely depending on the measuring conditions including the kinds of the fillers used and the kinds of the eluents used and the calculating methods so that it is difficult to obtain accurate average molecular weights, and therefore the determined values of the average molecular weights of the high molecular weight epoxy resins produced in these literatures are not entirely accurate.
Further, none of the literatures described above disclose that the obtained high molecular weight epoxy resins are able to be formed into film. Also, since the obtained epoxy resins are soluble in the solvents other than amide solvents, it is apparent that the methods disclosed in these literatures could not provide so called ultra high molecular weight epoxy resins which are linearly polymerized so highly as to have the capability of being formed into films having sufficient strength.
Furthermore, the conventional methods involve the problem that the production of high molecular weight epoxy resins in the conventional polymerization solvents takes a good long reaction time. In many of the working examples disclosed in tile literatures described above wherein ketone or ether solvents are used as the solvents, the polymerization reaction takes 10 to 24 hours, which is extremely longer than the reaction time of 1.5 to 10 hours taken by the polymerization in the absence of solvents.